The invention relates to a multifunction probe for aircraft. The piloting of any aircraft requires knowledge of its relative velocity with respect to the air, i.e. with respect to the relative wind. This velocity is determined with the aid of sensors for the static pressure Ps, the total pressure Pt and the angle of incidence α. α provides the direction of the velocity vector in a reference system, or frame, associated with the aircraft and (Pt-Ps) provides the modulus of this velocity vector. The three aerodynamic parameters therefore make it possible to determine the velocity vector of an airplane and, incidentally, of a tilting-rotor aircraft referred to as a convertible.
The various sensors for measuring static pressure, total pressure and incidence may be combined in a so-called multifunction probe. This probe may be mobile, like the one described in French Patent FR 2 665 539. It will then have a vane which is mobile about an axis perpendicular to the skin of the aircraft on which the probe is fitted. The mobile vane aligns naturally with the axis of the air flow around the aircraft, and the angular position of the vane around the axis of rotation gives the local angle of incidence αloc of the probe. Furthermore, the sensor for total pressure Pt is for example produced by means of a tube, referred to as a Pitot tube, which is secured to the vane and opens facing the flow at one of the ends of the tube. The other end of the tube is substantially closed. A pressure sensor measures the pressure of the air at the bottom of the tube in the vicinity of its closed end. For the pressure measured by the sensor to optimally represent the total pressure Pt of the air flow around the aircraft, it is important for the Pitot tube to be outside of a boundary layer which develops in the air flow in the immediate vicinity of the skin of the aircraft. Inside this boundary layer, the closer one is to the skin of the aircraft, the more a measured value of total pressure taken there will approach the value of the static pressure, and the less it will be possible to determine the modulus of the velocity vector of the aircraft with precision. The thickness of the boundary layer depends on the shape of the skin of the aircraft, and especially on the distance between the nose of the aircraft and the probe. At the conventional positions where a multifunction probe is placed, for example, the thickness of the boundary layer is of the order of 7 to 8 cm on a wide body airplane. An optimum position of the Pitot tube is therefore when it protrudes by about 10 cm from the skin of the aircraft.
The multifunction probe furthermore has pressure pickups arranged on the lateral faces of the mobile vane, making it possible to measure the static pressure of the air flow around the aircraft. In contrast to the Pitot tube, the pressure pickups for measuring the static pressure, which are referred to as static pressure pickups, may lie inside the boundary layer. However, the pressure measured by these pressure pickups, which is denoted Ps, is different to the local static pressure denoted Ps loc which would prevail at the position where the probe is fixed on the skin of the aircraft if the probe was not there, that is to say without perturbation. Nevertheless, the local static pressure Ps loc can be calculated using a pressure coefficient Kp of the probe. More precisely, Kp depends on the shape of the vane and the position of the pressure pickups on the vane. The coefficient Kp can be defined in the following way:
  Kp  =            Ps      -      Psloc              Ptloc      -      Psloc      where Ptloc represents the total pressure measured in the static pressure pickups.
In order to calculate the local static pressure Psloc, it is therefore necessary to know the pressure coefficient Kp of the probe as well as the local total pressure Ptloc. The pressure coefficient Kp is determined by calibration in a wind tunnel. During this calibration, the boundary layer developed along the stream of the wind tunnel is thin enough for the static pressure pickups to be outside the boundary layer. In this case, the total pressure Ptloc prevailing at the static pickups is the same as the pressure Pt measured by the Pitot tube.
But if the static pressure pickups lie in the boundary layer, then the total pressure Ptloc at the static pickups is not equal to that observed in the Pitot tube, and the calibrations carried out in a wind tunnel are no longer usable.
One solution for overcoming this problem is to make the pressure coefficient Kp zero. In this way, the measured pressure gives the local static pressure Psloc directly, regardless of the value of the total pressure Ptloc at the static pressure pickups.
One effective way of reducing the pressure coefficient Kp to a value of about zero is to increase the flexure of the vane. The flexure λ of the vane is defined as the angle which the leading edge of the vane makes with a direction perpendicular to the skin of the aircraft, or in other words with the axis of rotation of the vane. Specifically, assuming the flow to be incompressible and the fluid forming the flow to be ideal, the pressure coefficient Kp can be expressed by the following equation:
  Kp  =      1    -                  (                  V          V∞                )            2      where V represents the velocity of the flow at the pressure pickups and V∞ represents the velocity of the flow upstream of the probe. The above equation defines the coefficient of the pressure Kp for a zero flexure. When the leading edge of the vane is inclined by a flexure λ, the coefficient Kp is expressed in the following way:
      Kp    =          1      -                                    V            2                    +                                    (              V∞xtgλ              )                        2                                    (                      V∞                          cos              ⁢                                                          ⁢              λ                                )                                whence      ⁢                          ⁢      Kp        =                  [                  1          -                                    (                              V                V∞                            )                        2                          ]            ⁢                        x          ⁡                      (                          cos              ⁢                                                          ⁢              λ                        )                          2            
The implication of this is that the pressure coefficient Kp will be closer to zero when the flexure λ is larger. In other words, the more inclined the leading edge of the vane is with respect to its axis of rotation, the less the vane will perturb the flow.
In view of the other constraint mentioned above, however, namely sufficient extension of the vane to support the Pitot tube outside the boundary layer, there would be a risk of obtaining a vane of which the length of the leading edge is too great. This would result in a oversized vane.
It is an object of the invention to overcome this drawback by providing a probe whose extension is sufficient, whose pressure coefficient Kp of the static pressure pickups is close to zero and whose size is reduced.